L or X

The main distinction is linked to the number of electrons that the ligand supplies to the metal s coordination sphere if it supplies a pair of electrons, it is a ligand of type L, whereas a ligand that supplies just one electron is of type X. However, some ligands can supply more than two electrons to the metal. This notation, introduced hy M. L. H. Green, is generalized to yield ligands of type L Xx. 

There is also a lone pair on the oxygen atom. We shall see later why CO binds preferentially through the carbon atom ( 1.5.2.4 and Chapter 3, 3.2.2). 

There are other cases in which the two electrons supplied by the ligand L form a bond between two atoms of that ligand, rather than a lone pair. This can be a r-bond, as in the ethylene molecule, or, more surprisingly, a a-bond, as in the dihydrogen molecule (1-2).  

In these examples, two atoms of the ligand are bound in an equivalent way to the metal centre. The hapticity of the ligand is said to be 2. This type ofbond is indicated by the Greek letter rj, the nomenclature used being -C2H4 or respectively (1-2). 

It should be noted that in some of the examples given above, the radical centre also possesses one or more lone pairs, so that one migjit have considered it to be an L-type ligand. However, the use of a lone pair for bond formation would lead to a complex with an unpaired electron on the metal (.L —M). This electronic structure is less stable than that in which the unpaired electron and a metal elearon are paired to form the metal-ligand bond ( X- -M). It can be seen that in this case, all the electrons are paired, either as bonding pairs or as lone pairs. 

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The methanol can be removed by heating gently in vacuo. Similar compounds can be made with other carboxylate groups, either by using this method or by heating the acetate with excess carboxylic acid. Treatment of the anhydrous carboxylate with various neutral ligands (L) or anionic donors (X-) forms Rh2(OCOR)4L2 and [Rh2(OCOR)4X2]2-, respectively. The colour of the adduct depends on the donor atom in L (or X) ... 

We noticed that the gelation of polymers by the redox method is promoted if 2-3 times the calculated molar ratio of thiosulfite is used (see half Equation 9 below). The gelation rate was very slow when x=l or x=4 (Table V). 

Instead of 3-D plots, traces of the binding isotherm surface through a plane parallel to the [L]/[I] plane (contour diagrams) or profile plots (traces through the p./[L] or /x/[l plane can be used to explain certain special conditions. 

Equation 6.19 predicts an increasing IC50 with increases in either L or x. In systems with low-efficacy... 

For either of the above cases the sum (Ai + Aq) may be obtained from each of two sources. This redundancy offers a valuable check as to whether the assumptions of the kinetic scheme are vahd. However, it is not true that the time dependence of either the A(l) or X fluorescence gives a separate value for Aq or A,. This information must be obtained from the ratio of the two signal amplitudes. 

Nonbridging ligands can aflFect the energy and apparently also the intensity of IT bands. In the dimers (NH3)5Ru (pyz)Ru L(NH3)4 (34) and (NH3)5Ru (pyz)Ru X(bipy)o 29, 30), the energy of the IT band increases as variations in L or X increase the energy asymmetry between the two ends. For the mixed-valence dimer (bipy)2ClRu(pyz)-RuCl(bipy)2 ESC A studies demonstrated that there are discrete Ru(II) and Ru(III) sites (36). In recent work, Callahan and Meyer (37) found an IT band for the ion (Amax 1300 nm, c = 450 in acetonitrile) which has the approximate band width and solvent dependence that were predicted by Hush. IR data are consistent with localized valences. The properties of this ion differ markedly from those of the Creutz and Taube ion (NH3)5Ru(pyz)Ru(NH3)s (33), and the differences are consistent with a much stronger metal-metal interaction in the pentaammine system (37). 

We end this section by discussing several ligands whose classification as L- or X-type can create difficulties. 

Finally, if the derived classification after performing the above transformations contains both an L and a Z function, the classification is reduced further by using the transformation LZ = X2, as described in Section 1.01.4.1. As a result of this final transformation, it becomes irrelevant as to whether priority is given to placing the positive charge for cations on either the L- or X-function, although the final classification may be obtained more directly by placing it initially on the L-function. 

The majority of useful solvents have donor properties, and the ligands to be coordinated to an acceptor molecule or to an acceptor ion will have to compete for coordination with solvent molecules. Such reactions in solution may be represented as replacement reactions of solvent molecules coordinated to ions or molecules by competitive ligands L or X which may be neutral or charged. The occurrance of replacement or ligand exchange reactions will depend on the relative donor properties of solvent molecules D and competitive ligands L or X towards the ion or molecule under consideration ... 

The formation of the anionic complex is likely to occur if the competitive ligand X has stronger donor properties than the solvent molecules D. This condition will be best met by use of an acceptor solvent, where no competition is provided for coordination of L or X by the properties of the competitive ligand. 

It is the number of occupied coordination sites on the transition-metal center. Thus, a L or X ligand occupies one site, a L2 ligand (and most often a LX ligand) occupies two sites, and the L3 or L2X ligands occupies 3 sites. When the complex is of the form MLnXp, the coordination number is ... 

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